This invention relates to an X-ray spectrometer having a curved crystal monochromator which diffracts a continuous X-ray beam to produce a monochromatic X-ray beam of a different desired wavelength which can be changed. More particularly, this invention relates to apparatus for performing XAFS (X-ray Absorption Fine Structure) measurements using such an X-ray spectrometer.
The XAFS apparatus can measure a fine structure of an X-ray absorption spectrum around the X-ray absorption edge of a sample material. The XAFS method is classified to EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure). The EXAFS is defined as a fine structure of absorption observed over a wide energy range, about 1 keV wide, higher than the X-ray absorption edge of a sample material, as well known in the art. On the other hand, the XANES is defined as a fine structure of absorption appearing in a narrower region near the X-ray absorption edge, i.e., within a range of about xc2x150 eV of the edge, which is becoming recently an noticeable technique. The XANES measurements can be carried out using the same apparatus as the EXAFS apparatus, therefore the name of xe2x80x9cXAFS apparatusxe2x80x9d has recently been used, instead of xe2x80x9cEXAFS apparatusxe2x80x9d, because the XAFS apparatus can perform both the XANES and EXAFS measurements. An X-ray spectrometer according to the present invention is usable for the XAFS apparatus.
The XAFS apparatus can diffract a continuous X-ray beam, with the use of a crystal monochromator, to produce a monochromatic X-ray beam of a different desired wavelength, and can measure X-ray absorption coefficients of a sample for various wavelengths. The crystal monochromator may be usually a curved crystal monochromator for obtaining higher intensities.
In the XAFS apparatus using the curved crystal monochromator, an X-ray source, the curved reflective surface of the monochromator and a receiving slit, disposed before a sample, are to be positioned always on a Rowland circle. The wavelength of a monochromatic X-ray beam focused on the sample changes as the angle of incidence of the source X-ray beam changes with respect to the monochromator. During a change of the angle of incidence, the three components described above must be positioned always on the Rowland circle. The XAFS apparatus of this type is disclosed, for example, in Japanese patent publication Nos. JP 4-370748 A (1992), JP 6-66736 A (1994) and JP 6-313757 A (1994), noting that these publications use the name of xe2x80x9cEXAFS apparatusxe2x80x9d.
In the field of the XAFS apparatus using the curved crystal monochromator, there have been developed various improvements on the movement control mechanism for positioning the X-ray source, the monochromator and the receiving slit always on the Rowland circle. Ordinary XAFS apparatus has the Rowland circle within a horizontal plane. On the other hand, a special XAFS apparatus has the Rowland circle within a vertical plane for liquid sample measurements, which is disclosed in, for example, Japanese patent publication Nos. JP 6-66738 A (1994) and JP 6-317545 A (1994).
In the field of the XAFS apparatus using the curved crystal monochromator, there is no apparatus, as far as the inventors know, in which the sample remains perfectly stationary during measurements of X-ray absorption spectra. In the prior-art XAFS apparatus, when the above-described three components change in relative positions for a change of the wavelength of a monochromatic X-ray beam focused on the sample, the position of the receiving slit and/or the direction of an X-ray beam travelling from the monochromator to the receiving slit are to change. In this case, the position and/or the direction of the sample, disposed behind the receiving slit, are to change. Some samples, however, require to be stationary and therefore the prior-art XAFS apparatus has not been usable such a sample.
Further, measurements for liquid samples would have special requirements. The above-mentioned prior-art XAFS apparatus for the liquid samples has an improved mechanism of movement control so as to hold the posture of a vessel for liquid samples always in the horizontal position, provided that the vessel for liquid samples is under a translational movement. While the improved mechanism has the advantage of maintaining the horizontal position of the vessel, it has a disadvantage that the angle of incidence of an X-ray beam focused on the liquid surface changes as the wavelength of the X-ray beam changes, resulting in a change of the irradiated area size on the liquid surface. It has also another disadvantage that the liquid surface would wave when the liquid sample translates.
Accordingly it is an object of the invention to provide an X-ray spectrometer which can produce a monochromatic X-ray beam of a desired wavelength with a receiving slit remaining stationary.
It is another object of the invention to provide apparatus for XAFS measurements including an X-ray irradiation system consisting of an X-ray spectrometer and an X-ray measurement system having two X-ray detectors, wherein the two systems can be managed independently.
An X-ray spectrometer according to this invention comprises an X-ray source; a curved crystal monochromator; a receiving slit; and a movement control mechanism in which the angle of incidence of a continuous X-ray beam from the X-ray source can be changed with respect to the monochromator so that a monochromatic X-ray beam of a different desired wavelength is focused on and taken out from the receiving slit, provided that the X-ray source, the monochromator and the receiving slit must be positioned always on a Rowland circle. In the spectrometer, the X-ray source and the curved crystal monochromator can be moved so that said angle of incidence changes while the receiving slit remains stationary. Even when the angle of incidence of the X-ray beam changes with respect to the monochromator, the receiving slit remains always stationary and the direction of an X-ray path from the center of the curved crystal monochromator to the receiving slit remains always constant. Using this movement control mechanism, a monochromatic X-ray beam is taken out from the receiving slit, which is always at the same position, with the constant direction even when the wavelength of the taken-out X-ray beam changes. The X-ray spectrometer is usable as an X-ray irradiation system of XAFS apparatus, so that XAFS measurements require no movement of the sample and no movement of the X-ray detectors and therefore the X-ray irradiation system and the X-ray measurement system of the XAFS apparatus can be managed independently.
The X-ray spectrometer of this invention has a Rowland circle which can be arranged within a vertical plane. In this case, an X-ray source movement mechanism and a curved crystal monochromator movement mechanism can be supported by a horizontal long base which is positioned below the two movement mechanisms. With this support structure, the X-ray spectrometer can be compact as compared with the prior-art X-ray spectrometer which has a movement control mechanism supported by a comparatively large baseplate parallel to the Rowland circle. Using the support structure of this invention, the spectrometer includes a horizontal first slide which slides along first guide rail means fixed on the base, as a movement mechanism for monochromator; and a post pivotally mounted on the first slide and a second slide which slides along second guide rail means fixed to the post, as a movement mechanism for X-ray source. The curved crystal monochromator is set so as to move along with the first slide and the X-ray source is fixed to the second slide.